A conventional photonic crystal slab is configured by a two-dimensional photonic crystal which has multiple columnar portions (refractive index=v1) arranged in a two-dimensional cycle in a slab of a fixed refractive index (refractive index=v2), a lower cladding and an upper cladding (refractive index=v3) of a lower refractive index than v1, v2, which sandwich the two-dimensional photonic crystal in a film thickness direction and a substrate.
(1) Light propagating in the photonic crystal slab is confined in a vertical direction by total reflection at a boundary with the upper and lower claddings of the refractive index lower than any part of the two-dimensional photonic crystal (refer to Japanese Patent Laid-Open No. 2001-337236 (FIG. 1 for instance) for instance).
(2) There are also the cases where the two-dimensional and three-dimensional photonic crystals change cycle length and a direction of periodicity gradually or stepwise as to a position in the crystal (refer to Japanese Patent Laid-Open No. 2001-91701 (FIG. 9 for instance) for instance). FIG. 9 shows a configuration in which a high refractive index material 10 (SiO2) and a low refractive index material 11 (SiO2) are arranged to overlap alternately in a Z direction. FIG. 10 shows a principle of waveguide action in the configuration of FIG. 9.
All the disclosures of the documents of Japanese Patent Laid-Open No. 2001-337236 and Japanese Patent Laid-Open No. 2001-91701 are cited in its entirety and thereby become an integral part hereof.
(1) In the former case, however, it is necessary to select a material which satisfies a relation of v1≠v2, v1>v3, v2>v3 among the columnar portions (refractive index=v1) arranged in the two-dimensional cycle, other portions than columnar portions (refractive index=v2) and cladding portions (refractive index=v3). To reduce leakage of the light in the film thickness direction, a refractive index difference between v1 and v3 and the refractive index difference between v2 and v3 should preferably be large. To configure the photonic crystals, the refractive index difference between v1 and v2 should also be as large as possible.
For this reason, combinations of materials are limited.
For instance, in the case where the air of which refractive index is lowest (refractive index=1) is selected as the cladding, the photonic crystal portion has the high refractive index material (a semi-conducting material of refractive index=3 or more for instance) and the low refractive index material (resin, glass or air for instance) combined therein. Thus, it is possible to secure a minimum refractive index difference as the photonic crystal (a photonic bandgap can be obtained by using the air as the low refractive index material, in which case v1>v3 is not exactly satisfied).
However, an air bridge structure which uses the air as the cladding requires the photonic crystal to float in the air, and so its handling can be difficult.
In the case of using a fluorine compound (refractive index=approximately 1.3) which is the lowest solid refractive index material as the cladding, functions as the photonic crystal are limited when a semi-conducting material of a refractive index close to the high refractive index material is used as the low refractive index material of the photonic crystal portion. When a material of a refractive index of 2 or less such as resin or glass is used as the low refractive index material of the photonic crystal portion, the refractive index difference from the cladding becomes smaller and confinement of the light in the film thickness direction becomes weaker (in the case of the two-dimensional photonic crystal of which film thickness is 5 μm or less in particular, diffraction is so large that the light mostly leaks).
(2) In the latter case, a core of a long-period photonic crystal portion and the cladding of a short-period photonic crystal which surrounds the core are used to increase phase velocity in surrounding directions by periodic control so as to confine the light to the core of relatively low phase velocity.
This method can control the state of a propagation mode field of the light in a y direction freely by controlling the period, which is very advantageous in terms of device design. As the core and cladding are configured by solids, they are easy to handle and practical.
However, this method which changes the period requires sophisticated control in a manufacturing process, and also renders a manufacturing apparatus particular.
In consideration of the conventional problems, an object of the present invention is to provide an optical device capable of confining the light in a direction which has no period of the photonic crystal with a simpler optical system and a manufacturing method of the optical device for instance.